How reliable are UV indexes? Genuine question. The iOS weather app is far from 100% reliable and I wonder the margin of error regarding the UV index number it provides.
Basically it takes into account things like ozone measurements from satellites, latitude, forecasted cloud cover, and distance from sun (time of year).
See section “Calculating the UV index” here:
https://www.epa.gov/sunsafety/learn-about-uv-index
It’s not intended to be realtime. It’s more about understanding, generally, what the risk is.
A sibling comment mentions the ozone layer, but I severely doubt it varies enough to be a source of inaccuracy.
“Solar urticaria is a rare condition where the skin reacts to specific wavelengths of light rather than the overall UV intensity. The UV index is a general measure of the total amount of erythema-causing UV radiation (mainly UVB) that can cause sunburn in the average person.
But in solar urticaria, the trigger might be UVA, visible light, or even a narrow band of wavelengths — and the UV index doesn’t capture that nuance.
So it’s not that the forecast is wrong — just that the UV index isn’t designed to reflect the sensitivity profile of solar urticaria.”
In other words, you’re (literally) a special case. :)